This invention is generally directed to a process for rapidly charging uncharged toner particles, and more specifically, the present invention is directed to an improved process for rapidly charging uncharged toner particles added to a charged developer composition. In one embodiment of the present invention, there is provided a process for rapidly charging uncharged toner particles by incorporating into the developer mixture carrier particles containing a coating thereover which is embedded with certain materials such as fluoropolymers. Accordingly, in accordance with the process of the present invention, uncharged toner particles will acquire a charge of the appropriate intensity, and polarity, within a period of less than about five minutes.
The electrostatographic process and more specifically the xerographic process, is well known, as documented in several prior art references. In these processes, an electrostatic latent image is developed by applying toner particles thereto, using for example, cascade development, magnetic brush development, powder cloud development, and the like. For acceptable development to occur, the toner particles must be triboelectrically charged to an appropriate charge intensity, and to the desired polarity, either positive or negative depending for example, on the charge residing on the photoreceptor surface. With recently developed layered photoresponsive devices containing photogenerating layers, and charge transport layers, the surface of the device is charged negatively, necessitating the use of toner particles charged positively. Toner compositions can be prepared so as to assume a positive triboelectric charge by incorporating into the developer compositions involved, various charge enhancing additives including alkylpyridinium halides, various quaternary ammonium compounds, organic sulfates, organic sulfonates, and the like.
Moreover, it is known that new replenishment toner particles added to a developer composition contained in a commercial electrophotographic device, have essentially neutral average charge. Thus this toner must be charged to an appropriate level, which generally involves substantial mixing of the replenishment toner particles with the developer composition for a period of time equal to or greater than about 15 minutes. This time delay is undesirable as the replenishment toner particles can be printed out as background and the electrical properties of the developer composition can be adversely affected to the extent that images of low quality and low resolution result, and in some instances, development does not occur. Also, replenishment toner particles not charged to the appropriate polarity and magnitude can cause contamination as a result of the deposition of such particles on machine parts, thereby eventually resulting in the failure of charging corotrons, filters, and the like.
There is described in U.S. Pat. No. 4,304,830, the disclosure of which is totally incorporated herein by reference, a process for rapidly charging uncharged toner particles added to a developer composition, containing toner particles and carrier particles, wherein there is included in the developer composition an alkylpyridinium compound. As indicated in this patent, rapid charging of uncharged toner particles or admix charging, is accomplished within 2 to 3 minutes, as compared to a charging time of greater than 15 minutes for the acquiring of charges for uncharged toner particles which did not contain the alkylpyridinium halide material.
Additionally there is described in U.S. Pat. No. 4,378,420, the disclosure of which is normally incorporated herein by reference, a process for rapidly charging uncharged replenishment toner particles to a positive polarity wherein there is incorporated in the developer composition carrier particles consisting of a core containing a polymer coating thereon, and having incorporated therein an additive such as a perfluorooctonic acid, 2,4-dinitrophenol, 2,4,6-trinitrophenol, and naphthalene sulfonic acids. According to the disclosure of this application the uncharged toner particles acquire a positive charge within a mixing period of from about 5 seconds to about 5 minutes, thereby resulting in substantially the same level of positive charge intensity for the uncharged toner particles as the charge intensity of the toner particles in the charged developer composition.
Furthermore, disclosed in U.S. Pat. No. 4,073,980 are treated carrier particles which can be used in developer mixtures for the purpose of increasing the useful life of the developer, and also to provide desired triboelectric properties. In accordance with the disclosure of this patent, there is adhered to the surface of the carrier particle a mixture of a perfluorocarboxylic acid or derivatives thereof, and a dry lubricant such as molybdenum sulfide. This coating was found to result in carriers having a longevity and an abrasion resistance which is significantly greater than untreated carrier particles, or carrier particles coated with perfluorocarboxylic acid itself, and further the triboelectric properties of the developer composition containing such carriers was vastly improved over particles coated with similar materials.
In U.S. Pat. No. 3,922,381, there is disclosed carrier particles for use in electrophotographic processes, wherein the carrier particles contain a coating of a perfluorocarboxylic acid. With these coatings there results carrier particles having a longevity which is significantly greater than untreated carrier particles, or carrier particles coated with various other polymers. This patent further discloses that the resulting carrier particles are capable of imparting a positive triboelectric charge to electroscopic powders mixed therewith.
In U.S. Pat. No. 3,922,382, there is disclosed a method for selectively controlling the magnitude of the triboelectric characteristics of carrier particles for use in developer compositions, wherein the carrier particles containing various coatings, such as fluoropolymers are heated to certain temperatures. According to the disclosure of this patent, a coating containing a mixture of a fluoropolymer and a modifying material is effective in obtaining the desired polarity and magnitude of triboelectric charge on the toner particles and the carrier particles.
While the U.S. patents referred to herein describe rapid admix charging, there continues to be a need for improved processes wherein new uncharged replenishment toner particles added to a charged developer composition can acquire charge at the appropriate level and magnitude within a period of mixing time, that is less than about 5 minutes. This mixing time is referred to herein as fast, or rapid admix charging. Additionally, it would be desirable to accomplish rapid admix charging without introducing chemically active materials such as complex charge control additives into the developer composition, or more specifically into the toner resins.